1. Field of the Invention
This invention relates to a vane motor for use in dental and medical handpieces.
2. Background Art
The common dental handpiece is familiar--sometimes painfully so--to almost everyone who has ever been to a dentist. These handpieces usually include a pneumatic, rotary motor that drives some tool such as a drill bit or burr to allow the dentist to drill into or reduce a tooth. Similar devices also find use in medical contexts, such when drilling into or reducing bone, for example, in order to prepare a proper surface for attaching artificial joint members, for inserting anchoring pins, and so on.
One design goal for most handpieces is that their motors should be able to develop enough torque to drive the tool at a high enough rate of rotation, even under load. In one common type of handpiece, a turbine is mounted on a rotating shaft within the head of the handpiece. A disadvantage of an air-driven turbine is that its torque output is relatively low. Moreover, not only do turbines have a no-load speed that is needlessly high, but their speed also drops drastically when loaded.
Another common type of air-driven motor uses vanes instead of a turbine. In these devices, mainly flat, radial vanes are mounted to extend outward from a rotating shaft in planes that intersect the axis of rotation of the shaft. These vane motors in general deliver more torque than turbines, especially at the low rpm's normally preferred for cutting, but they typically have a much more complicated structure than turbines. For example, the rotating shaft on which the vanes are mounted is typically is mounted eccentrically, which means that the vanes must be able to move radially in the shaft as they turn. Many vane motors then require springs or other arrangements to ensure that the vanes extend fully where they are supposed to. This leads to other problems, such as the need to reduce friction between the vanes and the motor housing without losing power because of gaps.
Another goal is that the handpiece should not be too bulky or heavy. Here, the extra structures normally required in a vane motor work to particular disadvantage. One solution, both with turbine and vane motors, has been to mount the motor in the handle of the handpiece. A transmission system must, however, then be included from the motor to the head of the handpiece in order to drive the shaft on which the cutting or drilling tool is mounted. This transmission of course leads in turn to even more weight and bulk, and to reduced power.
In order to fit the motor in the tight space available in a dental or medical handpiece, the air intake and outlet ports must often be arranged in way that reduces the force the air can apply to the vanes, or that reduces the duty stroke of the vanes, or both. These port arrangements are, moreover, typically open into the end of the motor housing, that is, such that the intake air is directed along the vanes, not directly against them. Not only does this reduce the force of the air on the vanes, but it also mean that the motor housing itself must be precisely keyed so that the ports are properly aligned when the housing is installed into its opening in the handpiece.
The two main goals--high torque and compactness--typically conflict. One could, for example, increase the size of the air motor, but if the handpiece then becomes too heavy for the dentist to hold comfortably for the time it takes, for example, to do a tooth reduction, then achieving the extra power will not have been worth the effort.
Sometimes, vane motors require structures that make it harder for them to meet both design goals. In some known vane motors, for example, separate cylindrical members are included between the vanes and the inner wall of the handpiece housing, in order to reduce friction and wear on the vanes. In some cases, the outer tips of the vanes are keyed into this cylinder so that the cylinder rotates along with the vanes. This added part not only increases bulk and weight, and thus puts more stress on bearings. Furthermore, by increasing the inertia of the rotating parts of the motor, it reduces the power available for cutting.
Because of the extra complexity and weight of known vane motors, no vane motors now in use in dental or medical handpieces are even able to include the motors within the head of the handpiece. In other words, it has not been possible to make use of the high torque available from vane motors in a device that is compact and comfortable enough for dentists and surgeons to actually want to use in practice.
Still another problem common in existing motors is that they are not self-starting. This is generally a result of the other shortcomings of conventional motors--friction, inertia, inefficient porting, and so on. Whatever the cause, however, the solution is the same: yet another structure must be built in to start the motor. This of course simply makes the problem of excess bulk and complication even worse.
There are many known devices that attempt to achieve one or the other of these goals, or to find some suitable compromise. Representative conventional pneumatic motors used in dental handpieces are described in, for example, the following U.S. patents:
U.S. Pat. No. 4,120,623 (Lohn, Oct. 17, 1978); PA1 U.S. Pat. No. 4,175,393 (Frank, Nov. 27, 1979); PA1 U.S. Pat. No. 4,177,024 (Lohn, Dec. 4, 1979); PA1 U.S. Pat. No. 4,225,308 (Lohn, Sep. 30, 1980); PA1 U.S. Pat. No. 4,278,427 (Lingenhole, et al., Jul. 14, 1981); PA1 U.S. Pat. No. 4,403,958 (Lohn, Sep. 13, 1983); PA1 U.S. Pat. No. 4,740,144 (Biek, Apr. 26, 1988); and PA1 U.S. Pat. No. 5,064,361 (Kristof, et al., Nov. 12, 1991).
These known devices all suffer to some extent from insufficient torque or bulkiness, or from any of the several other problems associated with vane motors mentioned above. What is needed is a motor for dental and medical handpieces that doesn't, or at least not to the same extent.